Tides are controlled by the effect the moon’s gravity has on the ocean and water bodies near the ocean.

According to Princeton University’s Web site, it’s as if the ocean were bulging out toward and away from the moon. The moon pulls water toward it, and this causes the bulge toward the moon. Then you have a bulge on the opposite of the earth, away from the moon, which is caused by the moon pulling the earth away from the water on that side.

Because the moon and the earth move, this bulge moves, too, from one end of the earth to another (simply put). Think about what would happen if you put a marble-sized piece of metal under a sheet and then used a magnet to move the metal under the sheet without actually touching it. As the high point moves north, say, the sheet on the south end will begin to settle back down.

It takes time for this force to move around the earth. So when the force is greatest at Masonboro Inlet, say, that bulge will take four hours 38 minutes to get to Castle Hayne on the Northeast Cape Fear. Of course, there will be variations depending on the phase of the moon, the slope of the land around the water and other factors.

The twin bulges and the moon’s rotation mean that a local body of water experiences a high tide every 12 hours and 25 minutes or so.

Of course, this explanation is over-simplified. I encourage you to check out these Web sites for a more scientific explanation on this fascinating phenomena:

One Response to “ Why are the tides at Castle Hayne opposite most of the area beaches?”

On January 25, 2010 at 10:29 am David Staeblerwrote:

That’s a nice description for large oceans, but for estuaries (rivers and creeks with tides) there is more to the story. For these the only significant driving force for tides is the tidal ups and downs at its mouth, where it enters the ocean. When the tide is high at its mouth, then water flows upstream. When the tide is low, water flows out. This creates what I would describe as a slow moving gentle “wave” that’s goes up the river. It’s kind of like shaking a rope a one end; you can send waves up the rope.

So the timing is really about how long that wave takes to move up the river, which has primarily due to it’s length, including bends and curves that make the total distance much longer that the crow flies. This can explain why it takes around 2 hours to get to Wilmington, and more than two more hours for it to move further up to Castle Hayne. The same description can be made for creeks like Town Creek, where the driving force is the tide at its mouth into the Cape Fear River. I have not found a good online for this subject, but got my start in the book “The Tides” by Edward P. Clancy, 1968, Doubleday, which has a nice description in Chapter VII.

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